1. Field of the Invention
The present invention relates to a semiconductor device using a thin film transistor (hereinafter referred to as TFT) employing a semiconductor film formed on a substrate. Note that in this specification, the term “semiconductor device” indicates all the devices functioning by utilizing their semiconductor characteristics. Further, the semiconductor device manufactured in accordance with the present invention includes a display device represented by a liquid crystal display device in which a TFT is built-in and a semiconductor integrated circuit (a microprocessor, a signal processing circuit, a high frequency circuit, and the like) under the category.
2. Description of the Related Art
Development of information communication technologies has progressed and a display device as means for receiving information has been shifting from a CRT (cathode ray tube) to a flat panel display. This is because the CRT, which has been conventionally utilized for television display to provide various information, cannot sufficiently cope with the recent increased volume of information (for example, higher quality picture). Further, there also arises a problem in that it cannot sufficiently cope with high resolution for displaying a high quality picture, nor with enlargement of the screen. For example, when enlargement of the screen is to be advanced, the weight of the CRT itself becomes so great that it cannot be easily carried. Further, even in the same screen dimensions, when the resolution is made high, the luminance is degraded and the depth has to be extended. Therefore, installment of the device at home is rather restricted.
Then, as a candidate for a display device which can meet the demand for higher resolution and enlargement of the screen, a flat panel display characterized by being small in size, light-weight, and allowing saving of space is attracting attention. Particularly, a liquid crystal display device has been focused on and research development has progressed on a large scale.
In order to cope with the increased volume of information, the device must be able to write data in a short period of time. Further, in view of saving space and making the device small in scale, a display device is required to build-in a driving circuit. In order to realize such a display device, a TFT for forming a switching element and a driving circuit of a pixel needs to operate at high speed.
As a method of realizing a high speed operation of the TFT for example, there are considered a method in which a semiconductor layer is made polycrystalline instead of amorphous, and a Dual Gate structure described in JP 2737780 B in which a pair of gate electrodes sandwich the semiconductor layer.
However, although the TFT is formed by using a polycrystalline silicon, its field-effect mobility is equal to or lower than 1/10 that of a single crystal silicon, for example, and the electric characteristics thereof are not as good as the characteristics of a MOS transistor formed on the single crystal silicon substrate after all. Further, there arises a new problem in that an OFF-current is increased due to a defect formed on grain boundary.
Moreover, when an integrated circuit is formed by using the TFT, a threshold voltage (Vth) needs to be controlled in order to obtain a desired switching operation. The threshold voltage (Vth) is an important parameter for expressing switching characteristics of the TFT. When this value is shifted from a desired value, it causes a trouble in circuit operation. Therefore, in order to control the threshold, for example, in the case of an n-channel TFT, there is a problem in that the value is shifted to the minus side and a normally-ON state (a state of ON without applying a gate voltage) occurs as a result. In order to prevent this, there is adopted means for shifting the threshold voltage to the plus side by adding an impurity (acceptor) for imparting p-type conductivity in a channel forming region is taken.
Furthermore, a data line side driving circuit is required to prevent deterioration due to a high driving ability (ON-current, Ion) and hot carrier effect to thereby improve reliability. On the other hand, in order to obtain a high quality picture, a switching element of a pixel portion needs to have a low OFF-current (Ioff). As described above, in order to satisfy the demands for the liquid crystal display device, it is important to realize a TFT having characteristics required for the respective circuits.
Conventionally, the threshold control has been performed by adding an impurity element at a low concentration to the channel forming region. However, in the case of the structure in which a pair of gate electrodes sandwich the semiconductor layer, there is a problem in that a possibility of a carrier being caused on an interface between the semiconductor layer and an insulating film is high, the carrier is injected into the insulating film or the interface between the insulating film and the semiconductor layer, and thus the threshold rises. Further, in accordance with an energy band structure of this channel forming region, a path of the carrier exists only in the vicinity of the interface between the semiconductor layer and the insulating film. Therefore, there is a serious problem in that the hot carrier accelerated due to the voltage applied to the drain is injected into the insulating film or the interface between the insulating film and the semiconductor layer, and thus the mobility and a drain current are lowered.